drop mock dependency

mock is not used anywhere in the module, let's drop it.
This patch will help to hit this bug:
https://launchpad.net/bugs/1492636

Change-Id: I83b468e97eabfac5bb98093c2461ff0645720a90

Gemfile

@@ -13,7 +13,6 @@ group :development, :test do
   gem 'puppet-lint-variable_contains_upcase', :require => 'false'
   gem 'puppet-lint-numericvariable',          :require => 'false'
   gem 'json',                                 :require => 'false'
-  gem 'webmock',                              :require => 'false'
   gem 'netaddr',                              :require => 'false'
 end